(1) Field of the Invention
This invention relates generally to amplifiers and relates more specifically to common-mode control methods.
(2) Description of the Prior Art
Differential circuit implementations are widely used for realizing mixed-signal systems owing to their ability to substantially reject (as common-mode) ambient noise signals e.g. supply noise, reference noise and substrate noise that can ruin signal integrity in single-ended circuit implementations. An issue to address when designing fully differential amplifiers is designing a common-mode voltage control loop to set the differential output common-mode voltage ((see Sansen book reference, chapter 8: “Fully differential amplifiers”).
Common-mode control requires methods for both sensing and forcing the differential output voltages' common-mode voltage. Typical requirements for common-mode sensing are:                sense circuit linearity to avoid common-mode to differential signal conversion (see, e.g., VanPeteghem reference), and        ability to operate with a wide differential voltage output swing.        
These combined requirements are often met using a resistive averaging network to sense the common-mode voltage. Using a low resistance network can reduce the differential open-loop gain via output stage loading, which adversely affect Total Harmonic Distortion (THD), Power Supply Rejection Ratio (PSRR), etc., and can require significant extra power to drive. If buffers are used to minimize gain reduction then power dissipation increases further and differential output swing may be reduced (e.g. through the need to maintain constant gate-source voltage for a source follower buffer). These problems can be mitigated by using a high resistance network, but this may require significant additional silicon area in integrated circuit implementations. There is thus usually a trade-off between power, signal swing, and area when designing conventional common-mode feedback networks for fully differential amplifiers.
Differential amplifiers in chopper stabilized or switched capacitor circuits can often use the system clocks to implement low-power, low area, linear switched capacitor common-mode feedback (See Sansen, Shankar, Waltari and Hernandez-Garduno references for typical examples from the art). However, this is not an option in applications where a clock will not be routed to the amplifier, e.g. because one is not available (the system is continuous-time only) or it is not desired to introduce clocking artefacts into its signal path.
Multi-stage amplifiers can include feed-forward transconductance stages between the inputs and outputs (see Chen and Thandri references and Sansen book chapter 9 for a good overview of multi-stage amplifier compensation) for:                improved amplifier frequency compensation or        bandwidth extension by providing a transconductance path that bypasses the main amplifier at frequencies where the main amplifier cannot provide any gain (e.g. in dual path amplifiers), or extra current to drive load capacitances.        
FIG. 1 prior art shows a conventional continuous-time resistive common-mode-feedback (CMFB) arrangement.
Differential amplifier A1 requires some method to set the common-mode voltage (quiescent bias voltage) for the differential signals present at its inverted and non-inverted output terminals (nodes “Von” and “Vop” respectively in FIG. 1 prior art). Equal-valued resistors R1 and R2 create a voltage at node X that is the average of A1's differential output voltages:X=(Vop+Von)/2  (1)
Amplifier A2 compares voltage X with a common-mode reference voltage Vcmo and applies a feedback signal vcmfb to amplifier A1 in such a manner as to force voltage X to be controlled by voltage Vcmo (typically X being set equal to Vcmo). Thus the common-mode voltage of nodes Von and Vop is set to Vcmo.
This technique allows a large differential output voltage range (rail-to-rail) for amplifier A1. In comparison, some common-mode control methods use MOSFETs for common-mode sensing (see references), but this approach is usually much less linear and may limit A1's output voltage swing to avoid MOSFET turn-off. However, if resistors R1 and R2 are not to load the outputs of amplifier A1 (reducing the output stage gain) and are not to substantially increase the power dissipated when signal voltages are present at the amplifier outputs then large resistances (i.e. much larger than any load resistances on outputs Von and Vop) are needed. If large resistances are used then often large capacitors C1 and C2 are also required in parallel with R1 and R2 respectively to aid high frequency common-mode control loop stability. These large components can consume considerable silicon area.
FIG. 2 prior art shows a typical application of a differential amplifier (see Tauro reference) with internal CMFB details such as amplifier A2 omitted for clarity. In a balanced differential system, input impedances Zinp and Zinn are equal (=Zin), as are feedback impedances Zfbkp and Zfbkn (=Zfbk), giving system differential signal transfer functionH(jω)=−Zfbk(jω)/Zin(jω)  (2)    For resistive impedances, this gives the usual gain transfer functionGain=−Rfbk/Rin 
It is a challenge for engineers to adapt common-mode control methods to provide a feed-forward transconductance function additional to its common-mode voltage level control function in order to share the circuit implementation power and area overheads between these functions.
There are known patents or patent publications dealing with common-mode control methods.
U.S. Patent Publication (U.S. 2008/0315951 to Rysinski et al.) discloses a differential amplifier includes an output stage, a first common mode feedback circuit; and a current source. The output stage includes first and second complimentary output terminals. The first common mode feedback circuit is operable to determine an average voltage across the first and second complimentary output terminals. The current source is coupled to the output stage, and the common mode feedback circuit is operable to control the current source based on the average voltage. A method includes determining an average voltage across a positive output terminal and a negative output terminal of a differential amplifier output stage and controlling current injected into the output stage based on the average voltage.
U.S. Patent Publication (U.S. 2008/0246543 to Trifonov et al.) discloses a differential amplifier including a differential input pair coupled to a folded cascode stage and a common mode feedback circuit including a tracking circuit coupled to first and second outputs of the folded cascode stage. The first and second outputs are coupled to first terminals of first and second tracking capacitors which have second terminals on which a first common mode output signal is produced and also are coupled to first terminals of third and fourth tracking capacitors, respectively, which have second terminals on which a second common mode output signal is produced. The first and third tracking capacitors are discharged by first and second switches that directly couple the first and second outputs to first and second inputs of a common mode feedback amplifier. A desired common mode output voltage is applied to a third input of the common mode feedback amplifier. The switches are opened to cause the first and second common mode output voltages to be generated, causing a common mode feedback control signal to be generated for biasing the folded cascode stage.
U.S. Patent (U.S. Pat. No. 7,323,935 to Yang et al.) proposes a complementary transconductance amplifier having a common mode feedback circuit including a first-type transconductor, a second-type transconductor and a common mode feedback circuit. The first-type transconductor generates a first differential output signal pair in response to a differential input signal pair under the control of a first control signal. The second-type transconductor generates a second differential output signal pair in response to the differential input signal pair under the control of a second control signal. The common mode feedback circuit generates the second control signal in response to the first and second differential output signal pairs under the control of a common mode control signal.
Furthermore U.S. Patent (U.S. Pat. No. 5,084,683 to Nicollini) describes a filter comprising at least one completely differential operational amplifier having two inputs and two outputs and at least one pair of feedback circuits connecting said outputs with respective inputs of said amplifier outside of same. The operational amplifier has no common-mode feedback circuit, whose functions are performed by said feedback circuits external to the amplifier.
The following literature citations are known in the field of common mode control and operational amplifiers:                1. “Analog Design Essentials” by Willy M. C. Sansen, Springer 2006, ISBN-10 0-387-25746-2        2. “Design techniques for fully differential amplifiers”, J. Haspeslagh, W. Sansen, IEEE CICC 1988, pages 12.2.1-12.2.4.        3. “A general description of common-mode feedback in fully-differential amplifiers”, P. M. VanPeteghem, J. F. Duque-Carrillo, IEEE ISCAS 1990, pages 3209-3212.        4. “Feedback vs feedforward common-mode control: a comparative study”, J. M. Carrillo, J. L. Ausin, P. Merchan, J. F. Duque-Carrillo, IEEE ICECS 1998, pages 363-366.        5. “A low voltage operation transconductance amplifier using common mode feedforward for high frequency switched capacitor circuits”, A. Shankar, J. Silva-Martinez, E. Sanchez-Simencio, IEEE ISCAS 2001, pages 643-646.        6. “A switched-opamp with fast common mode feedback”, M. Waltari, K. Halonen, IEEE ICECS 1999, pages 1523-1525.        7. “Common-mode stability in fully differential voltage feedback CMOS amplifiers”, A. Tauro, C. Marzocca, F. Corsi, A. Di Giandomenico, IEEE ICECS 2003, pages 288-291.        8. “Continuous-time common-mode feedback for high-speed switched-capacitor networks”, D. Hernandez-Garduno, J. Silva-Martinez, IEEE JSSC Aug 2005, vol. 40, number 8, pages 1610-1617.        9. “A 0.5 V bulk-input operational transconductance amplifier with improved common-mode feedback”, M. Trakimas, S. Sonkusale, IEEE ISCAS 2007, pages 2224-2227.        10. “A power optimized continuous-time Delta-Sigma ADC for audio applications”, S. Pavan, N. Krishnapura, R. Pandarinathan, P. Sankar, IEEE JSSC Feb 2008, vol. 43, number 2, pages 351-360.        11. “Gain-enhanced feedforward path compensation technique for pole-zero cancellation at heavy capacitive loads”, P. K. Chan, Y. C. Chen, IEEE Trans Ccts & Syst II, Dec 2003, vol. 50, number 12, pages 933-941.        12. “A Robust feedforward compensation scheme for multistage operational transconductance amplifiers with no Miller capacitors”, B. K. Thandri, J. Silva-martinez, IEEE JSSC Feb 2003, vol. 38, number 2, pages 237-243.        